IPE does not respect panel settings / Requesting per renderer panel settings

Okay, so this IPE is fantastic and we're really excited to debut it to some clients.

We wanted to pare down the options of content_types on the panels IPE. Now normally, that would be done through the Panels Settings forms. The first thing that came up was that the panels IPE wasn't asking what content_types it was allowed to display. We see in line 21-23 of of panels_renderer_ipe.class.inc

 if (empty($this->display->cache_key)) {
     $this->cache = panels_edit_cache_get_default($this->display, NULL, NULL); // [EDITORS NOTE] Nulls for emphasis
}

which if you follow that down will return you with a default list of content types, i.e., all of them.

So now, our brain gears are turning and we're thinking that we could really use a per renderer display of settings. The use case here is that an admin who has access to the IPE will probably have less permissions than someone who has access to the entire backend of panels.

So here's a patch that implements a panels_ipe set of options. Thanks to populist for doing the work for setting up the panels_commons_form().

Now there are immediately some redflags for me, but I wanted to get the conversation started.

• cache_keys for panel_nodes are just the nid, this will lead to cache pollution between two people editing the same panel_node with two different interfaces. Now that's quite unlikely and we have the locking to prevent this, but I feel that expanding the cache_keys might be in order. Maybe something like $renderer_name:$module_name:$nid. Do you have a sense of how smart/hard this would be?
• Are there other things that we are missing? Setting allowable content_types leads me to believe there are other permission/settings that were overlooked.
• Is this the right place to be checking and setting the allowable_content_types